The present invention generally relates to-optical storage and data retrieval, and more particularly, to optical imaging techniques and configurations in a near-field optical storage system.
Optical storage can achieve high areal density data storage. The areal density of an optical storage device, in principle, may be limited only by the diffraction limit of spot size of an illuminating optical beam for reading or writing. Electro-optical data storage systems based on magneto-optical materials can be configured to produce an areal data density of up to or higher than about ten gigabits per square inch.
One approach to increase the areal data density in an optical storage system uses a reduced spot size. Due to the diffraction limit, a monochromatic optical beam can be focused to a significantly reduced or minimized spot size on the order of a wavelength. Light sources with short wavelengths, such as those toward the blue end of the optical spectrum, may be used to further decrease the spot size and thereby achieve an even higher areal density.
The numerical aperture of the objective lens may be increased to reduce the spot size at a given wavelength within the diffraction limit. This also increases the areal data density.
The present disclosure provides an electro-optical data storage system which includes an optical head for reading and writing and a head positioning system, an optics module with beam relay optics and signal detectors, an optical medium, and an electronic control system. The optical head is spaced from an active recording layer in the medium by a fraction of one wavelength of light to form a near-field configuration. The optical coupling between the head and the medium in such a near-field configuration can be effected by evanescent optical coupling. The areal data density, therefore, can be increased by focusing a read/write beam to a dimension smaller than the air-incident or free-space diffraction-limited minimum spot size.
The optical head may include one or more optical elements (e.g., an objective lens and a near-field lens) to effect a lens that has front and back focal planes. The optical head may be preferably suspended over the optical medium by a thin air-bearing surface to permit optical coupling to and from the optical medium by evanescent waves. The optical head may have a high numerical aperture up to or greater than unity for the optical head. In one implementation, the optical head includes an objective lens and a near-field lens that is spaced from each other at a fixed distance. The near-field lens may have a high index of refraction to achieve a desired numerical aperture. One embodiment of the near-field lens is a solid immersion lens.
The optical medium may be preferably structured to form a first surface recording configuration in which one or more active recording layers are formed on or near the top surface of the medium and spaced from the bottom of the flying head by a distance less than one wavelength of the optical source.
The optics module may be a fixed optics module in which the relative positions of different optical elements are fixed at predetermined distances. In one embodiment, the fixed optics module includes a light source, a data/servo detector, and beam control optics which may include a relay lens and an imaging lens to guide a read/write beam to and from the optical head.
A primary feature of the disclosure is to significantly reduce or minimize an asymmetric intensity distribution in the spot transmitted to and reflected from the medium and the read/write head.
Preferably, the imaging lens and the lens effected by the optical head (e.g., comprised of the objective lens and the near-field lens) form a telecentric imaging configuration in which a beam steering surface of a steering device is imaged to the front focal plane of the optical head closer to the imaging lens, i.e., the telecentric location. This configuration can reduce the effect of beam walk in the reflected beam from the optical medium which may be caused by beam scanning or other factors that affect the direction of the read/write beam incident to and reflected from the medium.
More preferably, the imaging lens and the optical form a pseudo telecentric imaging configuration in which the imaging plane is shifted from the telecentric location by a predetermined distance to reduce the asymmetric intensity distribution. Such asymmetric intensity distribution may be caused by beam walk and other adverse effects such as Fresnel reflections from the surfaces in the flying head and the optical medium, the total internal reflection from the near-field lens, beam clipping due to the limited aperture of the optical path in the flying head, and wavefront and ray aberrations.
An actuator in either rotary or linear configuration may be used as a coarse positioning means for the optical disk drive although other positioning devices may also be used. The fixed optics module and the flying head are attached to an actuator. Hence, any user data sector on the optical medium may be addressed with a read/write beam by adjusting the actuator. The beam control optics in the fixed optics module may include a beam-steering element such as a galvo mirror, a galvo-controlled prism or transparent plate to provide a fine positioning means for guiding the read/write beam.